Work will be concentrated in the following areas: 1. Platelet membrane markers; immunologic and immunochemical characterization. The variability of expression of HLA antigens on human platelets will be further studied with emphasis on its relationship to HLA phenotype, quantities of soluble HLA antigens in plasma, genetic control of HLA expression, and implications for platelet transfusion therapy. Studies will be conducted to determine whether similar variability exists in one or more lymphocyte subpopulations and, if so, to assess its immunobiologic significance. 2. Platelet heterogeneity and kinetics of circulating platelets. Kinetic studies of human platelet subpopulations and investigation of the biochemical and structural characteristics of such populations will be continued in an attempt to further characterize the relationship between platelet heterogeneity and platelet age. 3. Immune thrombocytopenias. Serologic and clinical studies of families in whom a child has developed isoimmune neonatal thrombocytopenic purpura will be conducted to further characterize the natural history of this disorder, the alloantigenic systems involved in its pathogenesis, and its response to therapy. Similar studies will be conducted in a large population with "acute" idiopathic thrombocytopenic purpura. 4. Short-term platelet preservation. Changes in membrane constituents of human platelets during short-term storage will be further characterized with special emphasis on major glycoproteins.